1. Field of the Invention
The present invention relates to ceramic oxide powder, a method for producing such ceramic oxide powder, ceramic paste produced using the ceramic oxide powder, and a method for producing such ceramic paste. In particular, the present invention relates to a method for producing ceramic oxide powder having a fine grain size and exhibiting a superior reaction property, by use of a combustion process while using citric acid as a combustion aid, and ceramic paste produced using such ceramic oxide powder.
2. Description of the Prior Art
Importantly, ceramic oxide powder, which is used as a raw material for various devices fabricated using ceramics, for example, inkjet heads, memory chips, and piezoelectric elements, should have a fine grain size and a uniform grain size distribution. This is because ceramic oxide powder having a fine grain size and a uniform grain size distribution can exhibit reduced activation energy after being surface treated and exhibit an enhanced reaction property and applicability in an electrically charged state.
Known methods used in the preparation of ceramic oxide powder are classified into a solid state reaction method(mixed oxide method) and a combustion method.
The solid state reaction method, which is also called a "mixed oxide method", is a method for producing ceramic oxide powder using an oxide or molten salt. In such a solid state reaction method, raw materials in the form of powder are mixed together and then subjected to a thermal treatment at a temperature of 1,000 to 1,200.degree. C. and then the resulting mixture is milled, thereby producing ceramic oxide powder. The preparation of ceramic oxide powder in the solid state reaction method is illustrated in FIG. 1a.
Typically, ceramic oxide powder produced in the solid state reaction method has a relatively large grain size of 0.2 to 2 .mu.m, even though depending on the grain size of raw materials used. For this reason, the solid state reaction method is unsuitable to obtain a grain size of 0.1 .mu.m. Furthermore, the solid state reaction method has a drawback in that it involves a heat treatment at a high temperature of 1,000.degree. C. or more.
In the combustion method, a homogeneous solution is prepared. The homogeneous solution is produced by dissolving a raw material in a solvent while adding a combustion aid thereto. The solution is then subjected to a thermal treatment at a temperature of 500.degree. C. or less, thereby causing a combustion reaction of the combustion aid. A primary product, which is produced by the combustion reaction, is subjected to an additional thermal treatment, so that it is crystallized. Thus, ultra-fine ceramic oxide powder is produced. The production of ceramic oxide powder according to the combustion method is illustrated in FIG. 1b.
As the combustion aid, an acid such as nitric acid or polyacrylic acid, or a nitrogen-contained compound such as glycine or urea may be used. An instantaneous ceramic producing reaction is carried out by virtue of high temperature heat of 1,000.degree. C. or more generated when the combustion aid is fired.
In order to obtain fine ceramic powder having a uniform grain size distribution, metallic alkoxide, nitrate, or acetate may be mainly used as a raw material. For a solvent used to dissolve such a raw material, an appropriate organic solvent or water is used.
The combustion method as conventionally used has an advantage in that fine powder having a grain size of 0.1 .mu.m or less can be produced at a low temperature of about 500.degree. C.
However, this conventional combustion method also has a drawback in that an intense combustion reaction involving an intense generation of flame and gas occurs, thereby resulting in a scattering of the produced powder. Furthermore, there is a problem of an excessively high volume-to-weight ratio due to an expansion of the combustion reaction product in volume.
The combustion reaction product typically has the form of an ash containing a large amount of unburnt carbon. That is, the combustion reaction product is non-crystalline, namely, amorphous. Therefore this combustion reaction product should be subjected to an additional thermal treatment at a temperature of 500.degree. C. or more for its crystallization.
Depending on the raw material used, intermediate products may be produced which have a self aggregation property, thereby exhibiting a high cohesion. Such intermediate products are problematic in that it is difficult to handle them. In particular, where citric acid is added, an intermediate product exhibiting a high cohesion is produced.
In the above mentioned conventional methods for use in the preparation of ceramic oxide powder, an excessive amount of acid is added. Therefore a base should also be added for a pH adjustment. In this case, however, there is a danger in that excessive heat may be generated due to a neutralizing reaction between the acid and base, thereby resulting in an explosion.
In the fabrication of various film devices using ceramics, a method has conventionally been used in which ceramic paste is produced from ceramic oxide powder, and then subjected to a thermal treatment in a state printed or molded on a vibrating plate.
Recent situations, in which it is important to provide micro and delicate structures of film devices, result in a demand for developments of a paste enabling printing or molding of micro patterns while exhibiting a uniform quality.
In conventional ceramic paste preparation methods, a mixture is prepared which consists of a binder for providing intrinsic properties of a paste, a vehicle for providing an appropriate flowability to produce a uniform paste and allowing the paste to be printed or molded, a plasticizer for allowing the paste to be formed into a micro structure, and a dispersant for providing a homogeneity to the paste. The mixture is dissolved in a solvent, thereby producing a solution. Ceramic powder, which are prepared by the solid state reaction method to have a mean grain size of 1 .mu.m, are added to the solution. The resulting mixture is then mixed.
An example of such conventional ceramic paste preparation methods is illustrated in FIG. 3.
In the illustrated case, a fibrous cellulose such as ethylcellulose is used for a binder. For a plasticizer, a phthalate is used. A terpineol is mainly used for a solvent.
Where ceramic oxide powder produced by the conventional solid state reaction method is used, it is essentially necessary to add organic materials as mentioned above. If there is no organic material added, it is then impossible to adjust the viscosity of the resulting ceramic paste. In particular, it is impossible to coat the ceramic paste over a vibrating plate.
The ceramic paste, which is produced by this method, cannot be formed into a desired shape at a low temperature because it contains ceramic grains of a large size. In association with a dispersant used, there is a problem in that use of the dispersant depends only on instructions of the supplier thereof without information about the composition or preparation method thereof.
In order to fabricate ceramic film, the ceramic paste is printed on a vibrating plate, dried at 130.degree. C., and then subjected to a heat treatment at 1,000.degree. C. or more. However, this method involves a problem in that an additional thermal treatment at a temperature of 500.degree. C. or more should be carried out after the drying process in order to achieve a binder removal for completely removing the added organic ingredients prior to the thermal treatment.
Furthermore, the thermal treatment, which is carried out at a temperature of 1,000.degree. C. results in a limited selection of usable vibrating plates.